Sediment volume partitioning, topset processes and clinoform architecture: understanding the role of sediment supply, sea level and delta types in shelf margin building and deepwater sand bypass : the Lance-Fox Hills-Lewis system in S. Wyoming
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This research focuses on how sediment supply, sea level and delta processes control the partitioning of the sediment budget across and into the topset, slope and basinfloor compartments of deepwater basins. Addressing this problem provides significant insight to characterize source-to-sink systems, improve tectono-stratigraphic models and predict sand bypass to deepwater areas. The research was carried out in the Lance-Fox Hills-Lewis shelf margin formed during the Maastrichtian in the Washakie-Great Divide basin of southern Wyoming. I use a database with approximately 520 wells integrated with outcrops to develop a high resolution, dynamic stratigraphy approach for shelfmargin characterization. The results emphasize the driving role of sediment supply in rapid shelf-margin building and deepwater sand emplacement. On the study margin, high sediment supply was able to outpace shelf accommodation even at times of relatively high and rising sea level. At these times, shelf margin clinoforms developed a more aggradational architecture with relatively thick and more marine influenced topsets formed in response to basin deepening due to rapid subsidence. The high supply and subsidence are interpreted to have resulted from crustal loading and significant erosion during prominent Laramide thrust-driven source uplift. The high supply caused the formation of highstand shelf-edge deltas with strong wave and river influences. These deltas resulted in extensive coastal sand belts at the shelf margin, and bypass of significant volumes of sand to deepwater areas. In contrast, during times of stable to very low rates of sea level rise, the basin developed more progradational clinoforms with more terrestrial and generally thinner topsets. More of the sediment was funneled to the basin floor and shelfedge deltas were under strong river and tidal influence. Stable or even falling sea level resulted from decreased subsidence or slight basin uplift, interpreted to have resulted from decreasing uplift, tectonic quiescence or possibly slight tectonic rebound in the basin. The Lewis-Fox Hills margin is considered supply-dominated, a term to denote moderately deep shelf margins (< 1000 m) that prograde at high rates (several tens of km/my) and deliver sand to deepwater areas recurrently and in large volumes even at sea level highstand.